The present invention relates to a variable displacement pump and, more particularly, to a pressure fluid utilizing equipment such as a power steering device for decreasing the force required to operate the steering wheel of a vehicle.
As a pump for a power steering device, generally, a displacement vane pump directly driven to rotate by a vehicle engine is used. In this displacement pump, the discharge flow rate increases or decreases in accordance with the rotational speed of the engine. A power steering device requires an auxiliary steering force which increases while the vehicle is stopped or is traveling at a low speed and decreases while the vehicle is traveling at a high speed. The characteristics of the displacement pump are contradictory to this auxiliary steering force. Accordingly, a displacement pump having a large volume must be used so that it can maintain a discharge flow rate necessary to produce a required auxiliary steering force even during low-speed driving with a low rotational speed. For high-speed driving with a high rotational speed, a flow control valve that controls the discharge flow rate to a predetermined value or less is indispensable. For these reasons, in the displacement pump, the number of constituent components increases, and the structure and path arrangement are complicated, inevitably leading to an increase in entire size and cost.
In order to solve these inconveniences of the displacement pump, variable displacement vane pumps each capable of decreasing the discharge flow rate per revolution (cc/rev) in proportion to an increase in rotational speed are proposed in, e.g., Japanese Patent Laid-Open Nos. 5-278622, 6-200883, 7-243385, 8-200239, and the like. According to these variable displacement pumps, a flow control valve is unnecessary unlike in a displacement pump. Waste of driving power is prevented to provide an excellent energy efficiency. No return flow to the tank occurs to prevent an increase in oil temperature. In addition, a leakage in the pump and accordingly a decrease in volumetric efficiency can be prevented.
An example of such a variable displacement vane pump will be described briefly with reference to FIGS. 25 to 27 showing the pump structure in Japanese Patent Laid-Open No. 8-200239 or the like. Referring to FIGS. 25 to 27, reference numeral 101 denotes a pump body; 101a, an adapter ring; and 102, a cam ring. The cam ring 102 is free to swing in an elliptic space 101b, formed in the adapter ring 101a of the pump body 101, through a swing fulcrum pin 102a serving as a support shaft. A spring means (compression coil spring 102b) biases the cam ring 102 to the left in FIGS. 25 to 27. A rotor 103 is accommodated in the cam ring 102 to be eccentric on one side to form a pump chamber 104 on the other side. When the rotor 103 is rotatably driven by an external drive source, vanes 103a held to be movable forward/backward in the radial direction are projected and retracted. Reference numeral 103b denotes a rotating shaft of the rotor 103. The rotor 103 is driven by the rotating shaft 103b to rotate in a direction indicated by an arrow in FIGS. 25 to 27.
First and second fluid pressure chambers 105 and 106 are formed on two sides around the cam ring 102 in the elliptic space 101b of the adapter ring 101a of the pump body 101, and serve as high- and low-pressure chambers, respectively. Paths 105a and 106a open to the chambers 105 and 106, respectively, through a spool type control valve 110 (to be described later), to guide as the control pressure for swinging the cam ring 102 the fluid pressures present upstream and downstream of a metering restrictor formed in a pump discharge path 111.
In this case, a variable metering restrictor 112 is formed of a hole 112a formed in the side wall surface of the pump body 101 that forms the second fluid pressure chamber 106, and a side edge 112b of the cam ring 102 that moves to open/close the hole 112a. Reference numeral 113 denotes a pump discharge path formed downstream of the variable metering restrictor 112.
When the fluid pressures of the pump discharge paths 111 and 113 present upstream and downstream of the variable metering restrictor 112 are introduced to the fluid pressure chambers 105 and 106 on the two sides of the cam ring 102, as described above, the cam ring 102 is swung in a required direction to change the volume in the pump chamber 104, as shown in FIGS. 25 and 26, thereby controlling the discharge flow rate in accordance with the flow rate on the pump discharge or outlet side, as shown in the flow rate curve shown in FIG. 28. In other words, the flow rate can be increased to a predetermined value by increasing the rotational speed of the pump, and is maintained at this value. When the rotational speed of the pump is high, the flow rate is decreased.
FIG. 25 shows a state that takes place from regions A to B in FIG. 28, and FIG. 26 shows a state that takes place from the region B to a region C in FIG. 28. In FIG. 26, the cam ring 102 swings to the right to restrict the variable metering restrictor 112. The pump discharge flow rate decreases in accordance with the restriction amount. When the variable metering restrictor 112 is restricted to the minimum position, the pump discharge flow rate is maintained at a predetermined value.
FIG. 27 shows a relief state in the region A of FIG. 28 wherein the pump is driven to rotate at a low speed. In this state, the pressure fluid utilizing equipment is actuated and the fluid pressure of the pump discharge side becomes a relief pressure. In the relief state in the region C of FIG. 28 wherein the pump is driven to rotate at a high speed, a relief valve 115 is open in FIG. 27 to control the relief flow rate in accordance with the open state of the variable metering restrictor 112.
In FIGS. 25 to 27, a pump suction opening (suction port) 107 is formed to oppose a pump suction region 104A of the pump chamber 104. A pump discharge opening (discharge port) 108 is formed to oppose a pump discharge region 104B of the pump chamber 104. These openings 107 and 108 are formed in at least corresponding ones of a pressure plate and a side plate (not shown) serving as stationary wall portions for holding pump constituent elements composed of the rotor 103 and cam ring 102 by sandwiching them from two sides.
The cam ring 102 is biased by the compression coil spring 102b from the fluid pressure chamber 106 and is urged in a direction to keep the volume in the pump chamber 104 maximum. A seal member 102c is placed in the outer peripheral portion of the cam ring 102 to define the fluid pressure chambers 105 and 106, together with the swing fulcrum pin 102a, on the right and left sides.
The spool type control valve 110 is actuated by differential pressures P1 and P2 obtained upstream and downstream of the variable metering restrictor 112, e.g., a metering orifice, formed between the pump discharge paths 111 and 113. The control valve 110 introduces a fluid pressure P3 corresponding to the magnitude of the pump discharge flow rate to the high-pressure fluid pressure chamber 105 outside the cam ring 102, to maintain a sufficiently large flow rate is maintained even immediately after the pump is started.
While the pressure fluid utilizing equipment (indicated by PS in FIGS. 25 to 27) is actuated to apply a load, when the differential pressures present upstream and downstream of the variable metering restrictor 112 become equal to or higher than a predetermined value, the control valve 110 introduces the fluid pressure P1 obtained upstream of the variable metering restrictor 112 as a control pressure to the high-pressure fluid pressure chamber 105 outside the cam ring 102, to prevent swing of the cam ring 102.
The pump body 101 is formed with a pump suction path 114 extending from a tank Ta to the pump suction region 104A of the pump chamber 104 through the low-pressure chamber of the spool type control valve 110.
The pump discharge path 113 is formed with the direct coupled type relief valve 115 serving as a pressure control valve. The relief valve 115 is formed at such a position that, when the pump discharge fluid pressure becomes equal to or higher than a predetermined value, it relieves the pressure fluid to the pump suction side (or tank Ta) through the pump suction path 114.
With this direct coupled type relief valve 115, during operation of the pump as shown in FIG. 27, when the pump discharge fluid pressure reaches a preset value or more, the flow of the fluid can be partly or entirely relieved to the pump suction side (tank Ta side). In particular, since the variable displacement pump does not have a flow control valve unlike in a displacement pump, the direct coupled type relief valve 115 is necessary to relieve the pressure fluid from the pump discharge side to the pump suction side.
In the conventional variable displacement pump having the above structure, when the pump is rotated at a low speed, the high-pressure (first) fluid pressure chamber 105 formed on one side of the cam ring 102 is set at the tank pressure, as shown in FIG. 25. Thus, an internal leakage inevitably increases particularly between the first and second fluid pressure chambers 105 and 106. More specifically, the pump discharge fluid pressure is introduced to the second fluid pressure chamber 106, to produce a large pressure difference between the second fluid pressure chamber 106 and the first fluid pressure chamber 105 which is set at the tank pressure. An internal leakage accordingly occurs around the swing fulcrum pin 102a that seals the fluid pressure chambers 105 and 106 from each other together with the seal member 102c.
The internal leakage includes leakage in the pump chamber 104 from the pump discharge region 104B to the first fluid pressure chamber 105 through the side surface of the cam ring 102, and leakage of the fluid pressures present upstream and downstream of the variable metering restrictor 112 guided to the two ends of the spool type control valve 110 over the lands of the spool to flow into the annular groove at the center of the spool where the tank pressure is introduced. Since the control valve 110 constantly controls a large pressure difference between the fluid pressure obtained upstream of the variable metering restrictor 112 and the tank pressure, an internal leakage cannot be avoided.
When such an internal leakage in the pump increases, the driving efficiency of the pump decreases. To avoid this, the portion where the internal leakage described above occurs must be machined with strict precision. This increases the manufacturing cost in turn.
In the conventional variable displacement pump described above, the control pressures acting on the fluid pressure chambers 105 and 106 on the two sides of the cam ring 102 to swing it are obtained by distributing the pump discharge fluid pressure and the tank pressure in accordance with the opening area of the lands of the spool in the control valve 110 to the path hole (path 105a) of the pump body 101.
In this control valve 110, as the control pressures increase, the area ratio increases. Then, the control valve 110 cannot sometimes follow this increase, and the characteristics of the pump rotational speed (N) with respect to the pump supply flow rate (Q) fluctuates to produce pulsation, as indicated by a broken line in FIG. 28. When this fluctuation occurs, the steering force may fluctuate in the power steering device, or noise such as fluid noise may be produced.
In order to improve the followability of the spool type control valve 110, particularly to allow smooth swing of the cam ring 102 moved by the fluid pressures controlled by the valve 110, the pressure difference between the first and second fluid pressure chambers 105 and 106 on the two sides of the cam ring 102 may be increased. According to the most general conventional structure, the pump discharge pressure is introduced to one fluid pressure chamber while the tank pressure is introduced to the other fluid pressure chamber. With this structure, however, the problem of internal leakage in the pump described above cannot be avoided.
Japanese Patent Laid-Open No. 9-273487 (corresponding to U.S. Pat. No. 5,895,209) proposes the following structure. A control valve for controlling swing of a cam ring is omitted. Fluid pressures present upstream and downstream of a metering restrictor directly act on the first and second fluid pressure chambers around the cam ring. On the inner surface of the cam ring, the position of a swing fulcrum pin is shifted in the circumferential direction from a range on which the pump discharge fluid pressure acts. This structure aims at balancing the pump discharge fluid pressures, that act on the cam ring, on the two sides of the swing fulcrum pin.
More specifically, in the variable displacement pump having the above structure, the fluid pressure, particularly, the pump discharge fluid pressure generates an unbalanced force between the pump suction and discharge opening positions of the pump chamber formed between the rotor and cam ring and the pin position serving as the swing fulcrum pin of the cam ring, i.e., the first and second fluid pressure chambers formed on the two sides of the cam ring. The pressure difference between the right and left sides is present in the pump chamber discharge regions corresponding to the first and second fluid pressure chambers. This pressure difference causes generation of a force for swinging the cam ring toward the second fluid pressure chamber (low pressure side), resulting in the unbalanced state. This pump, therefore, must have a structure which allows absorbing the above unbalanced force.
In this structure, various problems posed by pump machining, e.g., the machining precision and assembly precision of the respective portions of the pump, i.e., the cam ring, the swing fulcrum pin, the pump discharge opening that opens to the pump chamber, and the like are significant in obtaining an adequate swing motion of the cam ring about the swing fulcrum pin as the fulcrum, and machinability and assembly pose problems. If a low machining precision or assembly precision causes a manufacturing error, the swing motion of the cam ring about the swing fulcrum pin as the fulcrum may become unstable. If an unbalance occurs between the right and left sides of the cam ring about the swing fulcrum pin as the center, desired pump characteristics (flow rate characteristics) are difficult to obtain.
A structure is therefore sought for in which the problems accompanying the machining precision and the like are considered, the internal leakage as described above is solved, and the swing motion of the cam ring, particularly the return swing, can be performed smoothly, while the performance as the variable displacement pump can be effected.